The measurement of products or materials in tanks or silos is important for inventory control and custody transfer in many industries. Product level monitoring is also important in filling a storage device because overfilling and spillage of potentially hazardous products must be avoided. Similarly, level monitoring can be used to detect tank leakage; e.g., large unattended oil or gasoline storage tanks must be closely monitored for possible seepage which would cause contamination of soil and ground water.
Also, as the storage device is filled, the product level must be monitored to avoid overfilling and spillage of the potentially hazardous product. Large unattended oil storage tanks must be closely monitored for possible seepage which would cause contamination of the soil and ground water.
The standard practice of measuring the level in a storage vessel is to lower a stick or a rope into the vessel until the product is touched. By measuring the extended length the actual level of the product or material in the tank can be determined. This is usually called "sticking the tank". A quite common example of "sticking the tank" occurs when a gasoline station attendant measures the level of a gasoline storage tank by inserting a calibrated stick into the tank and removing it to see how much of its length is wet. Such a procedure is potentially a potentially very dangerous procedure and vents the tank contents to the atmosphere, which is undesirable. Although many of the newer measuring devices which use the stick or rope concept are automated and quite accurate, such devices typically require a physical intrusion into the storage environment. Such breaching of the tank raises the possibility of the contamination of the storage vessel contents and, depending on the corrosive nature of the vessel contents, could impair or destroy the measuring device.
Since the early 1970s, various level measuring devices using radar techniques have been developed to eliminate the necessity of the measuring device touching the storage vessel contents. Typical radar level measuring devices use frequency modulated continuous wave (FM-CW) signal wave forms in which the frequency of the transmitted signal is swept at a constant rate over about 1 gigahertz (GHz) around its center frequency. The reflected signal which, due to the time delay caused by the signal traveling from the transmitter to the storage vessel contents and traveling back to the receiver upon reflection from the contents' surface, will be at a different frequency than the presently transmitted signal which, during the transit time, was swept to a different frequency. The difference in frequency between the transmitted and received frequency is an indication of how far the initial transmitted signal had to travel, and thus the distance to the storage vessel contents can be determined. In such systems the minimum resolution of the measurement and the minimum measurable distance is directly related to the sweep width of the transmitted signal. Also, since the return signal is modulated by the chirp, the required detection bandwidth must be wide, decreasing the signal-to-noise ratio and thereby limiting the maximum range.
The U.S. Federal Communications Commission (FCC) and international regulations limit the maximum bandwidth of the transmitted signal (at the required power levels to overcome the path loss) to 250 megahertz (MHz). Current equipment, employing fixed sweep rates and fixed sweep lengths, cannot resolve or filter out fractions of return signal cycles. These fractions of cycles occur when the sweep, which is not synchronized with the return signal, reverses or resets before a complete cycle is received. Therefore, these systems exhibit significantly lower signal-to-noise ratios and require at least a 1 GHz bandwidth.
What is needed is a method and apparatus to provide an inexpensive and highly accurate measurement for the level of a product in a storage tank or silo using microwave signals given the FCC and international regulations constraint of maximum bandwidth of the transmitted signal of 250 MHz, in a system which can resolve closely spaced echoes and exhibit good signal-to-noise-ratio.